The traditional implantable cardiac pacemaker includes a pulse generator device to which one or more flexible elongate lead wires are coupled. The device is typically implanted in a subcutaneous pocket, remote from the heart, and each of the one or more lead wires extends therefrom to a corresponding electrode, coupled thereto and positioned at a pacing site, either endocardial or epicardial. Mechanical complications and/or MRI compatibility issues, which are sometimes associated with elongate lead wires and well known to those skilled in the art, have motivated the development of cardiac pacing devices that are wholly contained within a relatively compact package for implant in close proximity to the pacing site, for example, within the right ventricle (RV) of the heart. With reference to FIG. 1, such a device 100 is illustrated, wherein pace/sense electrodes 111, 112 are formed on an exterior surface of a shell 101 that hermetically contains pulse generator electronics and a power source. FIG. 1 further illustrates a fixation member 115 mounted to an end of shell 101, in proximity to electrode 111, in order to fix, or secure electrode 111 against the endocardial surface of RV, and electrode 112 offset distally from electrode 111. Shell 101 is preferably formed from a biocompatible and biostable metal such as titanium overlaid with an insulative layer, for example, medical grade polyurethane or silicone, except where electrode 112 is formed as an exposed portion of capsule 101. A hermetic feedthrough assembly (not shown), such as any known to those skilled in the art, couples electrode 111 to a pulse generator contained within shell 101.
Any one of a number of fixed shape or steerable delivery catheters, which are known in the art, can be employed to deliver device 100 into the heart for implantation, for example, by a transvenous femoral approach, via percutaneous entry. With reference to FIG. 2A, a delivery catheter 20 is shown extending up through the inferior vena cava IVC and into the right ventricle RV via passage through the right atrium RA and tricuspid valve. A distal portion 250 of catheter 20, in which device 100 is initially fitted for delivery into the RV, is shown pulled back from the implanted device 100, for example, to conduct an implant evaluation (i.e. sensing and pacing threshold testing). FIG. 2A illustrates a temporary tether 202 joining device 100 to catheter 20 so that, if results of the implant evaluation indicate that the implant site for device 100 should be adjusted, tether 202 may be used to pull device 100 back into distal portion 250 of catheter 20 for repositioning. Once device 100 is implanted at an appropriate site, tether 202 is separated therefrom, according to any suitable method known in the art. With reference to FIG. 2B, if subsequent explant of device 100 is necessary, a retrieval snare 224, such as any suitable type known to those skilled in the art, may be employed.